The apparatus for plasticating rubber, thermoset or thermoplastic materials includes a screw rotating within a long, heated barrel. For purposes herein plasticating shall include the processing of rubber. Material in the form of pellets, granules or powders to be processed enters the barrel through an inlet port, and is transported along the barrel by a rotating screw formed with flights, which push the material forward through the barrel. As the material moves through the barrel it is heated, worked and progressively converted to a melted or molten state. The melt or molten material is delivered under pressure through a restricted outlet or discharge port to make the finished article. It is desirable that the molten material leaving the apparatus be completely melted and homogeneously mixed, resulting in uniform temperature, viscosity, color and composition.
The barrel of the plasticating apparatus has the form of an elongated, generally circular, hollow cylinder having relatively thick walls, which are heated at various axial positions by heaters located on the outer surface of the barrel. The rotating screw, which extends longitudinally through the barrel, is responsible for forwarding, melting, pressurizing and homogenizing the material as it passes from the inlet port to the outlet port. Typically, the screw has a radially inner core. The core has a helical flight along its length, the flight and a cylindrical inner surface of the barrel defining a helical space in which the material is transported to the outlet.
In general, the plasticating screw has one or more sections along its axis, each section being designed for a particular function. Ordinarily, when plasticating thermoplastics, there is a feed section, a melting section and a metering section arranged in a series. The melting section is referred to interchangeably as the intermediate, compression or transition section. The feed section extends forward from the inlet port where rubber, thermoset or thermoplastic material is introduced into the apparatus and pushed forward by the screw along the interior of the barrel. The material is then worked and heated in the melting section. After approximately 40% to 80% of the material has been masticated or plasticized, the remaining material becomes randomly dispersed within the mix. But most melting initially occurs in the melting section at or near the heat source at the inner wall of the barrel.
Throughout the history of injection molding, the injection barrel has changed primarily to improved wear resistance. A barrel must contain the injection pressure, be wear resistant and transfer heat into the plastic resin from the heating source outside the injection barrel. The less resistant the barrel is to heat transfer, the faster the resin will reach melt and the closer the process temperature can be held to a set point.
A barrel for a plasticating process, namely extrusion, injection molding, blow molding or die cast machines typically are heated by electric heater bands or liquid flowing through jackets. To achieve the proper magnitude of heat over a wide area of the barrel surface, more than one heater band or jacket is normally required. Preferably a barrel used in the process is also cooled by conduction through the barrel wall during a portion of the plasticating process in order to lower the temperature of the molten material before it is transferred from the barrel into a downstream forming process, such as a mold or die. This cooling of the material while in the barrel helps to shorten the time required to form the material, thereby decreasing the cycle time.
Non-electric jackets are used for regulating the temperature of material being processed by passing a heat transfer medium through channels located in the barrel. The medium for carrying heat energy to and from the barrel is usually water, oil or steam, whose temperature is controlled outside the barrel by a boiler, fluid heater or chiller. The heat transfer medium can also be passed through jackets on the outer surface of a plasticating barrel, which is usually made of steel and therefore is subject to corrosion caused by the accumulation of the minerals in the heat transfer medium. This method of regulating the temperature of the processed material has several disadvantages including leakage of the medium, clogging of heat transfer channels with residue, and difficulty in sealing the channels. U.S. Pat. No. 5,363,900 describes a heat control system that includes a steel mold having a fluid channel and thermally conductive pins embedded in the mold and extending into the channel. Heat is conducted to the pins from the mold body and transferred through the pins to the fluid in the channel for cooling the mold.
A thermocouple, located on the barrel, produces a signal representing surface temperature. The signal is transmitted to a temperature controller, which controls the heater on the basis of a controller set point. Electric heaters have variations that can cause contact differences between the heater and the barrel. Usually the same amount of electric power is supplied to all the heaters subject to the control of the thermocouple signal, but each heater may supply a different magnitude of electric power to the barrel surface. If one heater, distant from the thermocouple, supplies less heat to the barrel surface than another heater located closer to the thermocouple and more closely contacting the barrel, it will transmit less heat to the barrel. These variables cause hot and cold spots in the process equipment and may adversely affect the processed material.
In order to avoid these process and equipment difficulties and to shorten process time, there is a need to minimize the resistance of the barrel wall to heat transfer. This can be accomplished by raising the overall coefficient of thermal conductivity of a mold, die, or barrel, or by raising the coefficient of thermal conductivity in a particular area of such devices where more uniform or higher thermal conductivity between the resin material being processed and its heating or cooling source is required.